A horse of ours is allergic to alfalfa (he's 29 now, been without alfalfa his whole life) When he was first bought as a two yr old, he was allergic to alfalfa. He had the nastiest reaction, that almost killed him!!! My mom's boyfriend had his hay guy turn one of his crops into an alfalfa free zone, and he would buy that for his horses. Well soon after that people found out about it and the hay guy ended up turning ALL of his hay fields into alfalfa free zones. He's one of the most popular sellers now!! But from his experience he has found out that an alfalfa is not always that beneficial.
I know of another horse who has been eating alfalfa hay alll its life, the horse got really sick last year and as soon as it was taken off of alfalfa it gradually started to get better. Maybe there is something alfalfa does to horses?